Every blog is different. Most are set up to allow none at all, many will only allow a few codes like anchor tags, and only very small few will allow almost anything. The ones that do, many times will often also require code tags similar to what is allowed when posting in BHW forums.
You just need to search g00gle to get an idea of the most common tags that are allowed, and if the blog has any kind of help page it might tell you what is allowed there. If you are going to do any kind of autoposting, it is best to leave out the tags since the most common is to not allow the html. Include your url including the http and some will strip it out, some will leave it as text, and some will automatically make it a link.
edit - thinking about it some more, I might be wrong about just including the url as text and letting the blog change it. A lot of blogs will change that to a link, but many will leave it as text. If you're wanting to build backlinks to your site, then you might want to put the url in your post with an 'a href' tag, if the blog doesn't allow html, but does change urls to links then the code will show up making your post look like crap, but it will still be a link. If it does allow code, but doesn't auto make urls into links, then you just made it one. Doing it that way will result in more true links, because it will always be a link except for the ones that strip the urls. I don't know if I just explained that very well. Hope you understand what I mean.
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